The role of prolactin in the regulation of clutch size and onset of incubation behavior in the American kestrel

In most bird species, the timing of incubation onset may influence the degr ee of hatching asynchrony, which, together with variation in clutch size, a ffects reproductive success. In some domesticated species that usually show no hatching asynchrony, plasma prolactin concentrations in females rise wi th the onset of incubation and the end of laying, and this rise enhances in cubation behavior and may terminate laying. To investigate whether a rise i n prolactin during laying is involved in the regulation of clutch size and incubation onset in a species with hatching asynchrony, we measured plasma concentrations of immunoreactive prolactin (ir-prolactin) in laying America n kestrels, Falco sparverius, and quantified clutch size and incubation beh avior. In a separate study, we administered one of three concentrations of ovine prolactin (o-prolactin) via osmotic pumps implanted in females when e gg 2 of a clutch was laid. ir-Prolactin concentrations during laying were h igher in small than in large clutches and increased in parallel with the de velopment of incubation behavior. o-Prolactin treatment enhanced incubation behavior, but did not affect clutch size, possibly because the manipulatio n was performed after clutch size had already been determined. Consistent w ith studies on domesticated species that show synchronous hatching, our res ults indicate that rising prolactin during laying enhances the expression o f incubation behavior in a species that shows hatching asynchrony. Further studies are necessary to determine whether the relationship between prolact in and clutch size in the American kestrel is one of causation or of mere a ssociation. (C) 2000 Academic Press.